The present invention relates generally to oscillator circuits. More particularly the invention relates to a lockproof digital circuit employing low cost digital inverters suitable for integrated circuit mass production. The circuit produces a logic level oscillation at a frequency dictated by an associated tank circuit. The signal excursions in the tank circuit can be maintained at a substantially low energy level notwithstanding the comparatively high energy level of the signal output. This greatly minimizes crosstalk and interference.
Resonant circuits are commonly used to measure physical parameters. For example, there is a class of displacement sensors which employs an inductance tank circuit with moving vane tuning plate to measure position. Our prior U.S. Pat. No. 4,644,570, entitled "Sensor Amplification and Enhancement Apparatus Using Digital Techniques," issued Feb. 17, 1987, describes such a displacement sensor. The position of the tuning plate alters the inductance of the coil and thus alters the resonant frequency of the tank circuit. By energizing the tank circuit and measuring the frequency of the resonant oscillations produced, position of the tuning plate can be inferred.
With the advent of digital technology, it has become desirable to use digital components in an oscillator circuit, since digital circuits can be made small and inexpensive and are easily implemented in integrated circuit (IC) packages. However, conventional digital circuits in such applications exhibit lockup in which the digital components assume conflicting states which prevent oscillation. This can be caused by signal transients and dropouts and represents a serious shortcoming of conventional designs.
Digital shaper circuits and hybrid analog/digital circuits have also been experimented with but difficulties in addition to lockup have been encountered. For instance, in switched circuit applications or in applications where electromagnetic radiation from the inductor interferes with other circuitry, these circuits pose problems due to the relatively high energy levels at which these circuits operate.
In a switched application, for example, it may be necessary to abruptly change the current applied to the inductor. Abrupt changes in current cause high energy voltage spikes which can cause radio frequency interference and which can induce errors in associated digital circuitry. Even in unswitched applications, high energy level oscillations in the tank circuit can cross-couple with and radiate to other parts of the circuit, causing unwanted crosstalk and interference.
The present invention overcomes the above limitations of conventional high level oscillator circuits by providing a fully digital lockproof circuit in which the signal amplitude in the tank circuit can be adjusted and kept at a low level. The oscillator circuit of the invention comprises a digital circuit employing first, second and third inverters connected in series. The three inverter digital circuit is operable between bistable high and low logic level states. A tank circuit comprising a capacitance and an inductance is coupled to the input of the first inverter to establish a resonant frequency. A negative feedback circuit connects the output of the third inverter with the input of the first inverter. The negative feedback circuit biases the digital circuit to an intermediate point between the high and low bistable states. The negative feedback circuit may include a frequency selective circuit for decoupling the negative feedback at the resonant frequency.
The circuit further includes a positive feedback circuit coupled between the output of the second inverter and the tank circuit. The positive feedback circuit delivers energy to the tank circuit, thereby inducing oscillation in the tank circuit at the resonant frequency. The output of the third inverter is a logic level signal which oscillates between the high and low bistable states in synchronism with the induced oscillation in the tank circuit. The positive feedback includes an impedance for restricting the amount of energy delivered to the tank circuit, thereby reducing the amplitude of the oscillations in the tank circuit, without reducing the signal amplitude at the output of the third inverter This has the advantage of greatly reducing the signal level in the inductance and thereby greatly reducing cross talk and interference problems.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.